{"record":{"id":"09b30b0f049cb8e3","repo":"apache/beam","slug":"unable-to-infer-the-types-of-startbundle","errorCode":null,"errorMessage":"Unable to infer the types of StartBundle","messagePattern":"Unable to infer the types of StartBundle","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"critical","filePath":"sdks/go/pkg/beam/register/register.go","lineNumber":242,"sourceCode":"\nfunc buildStartBundleWrapper[I0, I1, I2, I3, I4, I5, I6, I7, I8, I9 any](doFn any) func(any) reflectx.Func {\n\tstartBundleIn := -1\n\tstartBundleOut := -1\n\tstartBundleMethod := reflect.ValueOf(doFn).MethodByName(\"StartBundle\")\n\tif !startBundleMethod.IsValid() {\n\t\treturn nil\n\t}\n\tstartBundleIn = startBundleMethod.Type().NumIn()\n\tstartBundleOut = startBundleMethod.Type().NumOut()\n\tswitch {\n\n\tcase startBundleIn == 0:\n\t\tswitch {\n\t\tcase startBundleOut == 0:\n\t\t\tif _, ok := doFn.(startBundle0x0); ok {\n\t\t\t\treturn registerStartBundle0x0FuncAndMakeStructWrapper()\n\t\t\t} else {\n\t\t\t\tpanic(\"Unable to infer the types of StartBundle\")\n\t\t\t}\n\t\tcase startBundleOut == 1:\n\t\t\tif _, ok := doFn.(startBundle0x1[error]); ok {\n\t\t\t\treturn registerStartBundle0x1FuncAndMakeStructWrapper[error]()\n\t\t\t} else {\n\t\t\t\tpanic(\"Unable to infer the types of StartBundle\")\n\t\t\t}\n\t\tdefault:\n\t\t\tpanic(\"Invalid signature for StartBundle\")\n\t\t}\n\n\tcase startBundleIn == 1:\n\t\tswitch {\n\t\tcase startBundleOut == 0:\n\t\t\tif _, ok := doFn.(startBundle1x0[I9]); ok {\n\t\t\t\treturn registerStartBundle1x0FuncAndMakeStructWrapper[I9]()\n\t\t\t} else if _, ok := doFn.(startBundle1x0[context.Context]); ok {\n\t\t\t\treturn registerStartBundle1x0FuncAndMakeStructWrapper[context.Context]()","sourceCodeStart":224,"sourceCodeEnd":260,"githubUrl":"https://github.com/apache/beam/blob/12126d8942aaf848030c478b4c6a28c6af861c66/sdks/go/pkg/beam/register/register.go#L224-L260","documentation":"buildStartBundleWrapper determines the concrete StartBundle method shape of a DoFn at registration time. When no StartBundle parameters could be inferred (startBundleIn == 0) and the DoFn does not implement the expected startBundle0x0 interface shape, it panics \"Unable to infer the types of StartBundle\". This happens when a user-defined StartBundle method has a signature the registration reflection code cannot map to a known wrapper.","triggerScenarios":"Registering/using a DoFn whose StartBundle method matches none of the supported signatures: wrong parameter or return types (e.g. StartBundle(ctx) or StartBundle() with an unsupported return), or a generic instantiation that fails the startBundle0x0/startBundle0x1[error] type assertions.","commonSituations":"Typo'd or custom StartBundle signatures (extra params, missing context, wrong receiver generics); migrating DoFns across Beam versions where supported signatures changed; defining StartBundle on a value receiver vs pointer receiver mismatch so the type assertion fails.","solutions":["Change StartBundle to a supported signature, e.g. func(*MyFn) StartBundle() error or with a beam context type.","Return error from StartBundle if side effects can fail (startBundle0x1[error] shape).","Verify receiver matches how the DoFn is registered (pointer vs value) so the type assertion succeeds.","Check the beam SDK version's register.go for the exact list of supported StartBundle shapes."],"exampleFix":"// before\nfunc (f *MyFn) StartBundle(ctx context.Context) { ... } // not inferable\n\n// after\nfunc (f *MyFn) StartBundle() error {\n    // bundle setup\n    return nil\n}","handlingStrategy":"validation","validationCode":"// compile-time check the DoFn satisfies a supported shape\nvar _ interface{ StartBundle() error } = (*MyFn)(nil)","typeGuard":"func hasSupportedStartBundle(fn any) bool {\n    _, ok0 := fn.(interface{ StartBundle() })\n    _, ok1 := fn.(interface{ StartBundle() error })\n    return ok0 || ok1\n}","tryCatchPattern":null,"preventionTips":["Use only supported StartBundle signatures: () or () error.","Ensure receiver type matches registration (pointer vs value).","Add compile-time interface assertions for DoFn lifecycle methods."],"tags":["go","apache-beam","dofn","reflection","panic"],"backgroundTag":"unsupported-operation","analyzedSha":"12126d8942aaf848030c478b4c6a28c6af861c66","analyzedAt":"2026-09-13T01:50:10.254Z","contentChangedAt":"2026-09-13T01:50:10.254Z","schemaVersion":2},"datasetVersion":"2026-09-14T16:17:12.679Z"}